On a computer mother board, a RTC(Real-Time Clock)is provided to maintain the data stored in a CMOS on the mother board. Generally, electrical power is provided to a RTC by a battery arranged on the mother board. The battery can be a lithium cell, a nickel/hydrogen cell or a nickel/cadmium cell, an exterior cell box, or a RTC module of with an internal cell. The only method of extending the life of the power supply is to use a battery having excellent efficiency. Although the power consumption of the CMOS is low, the battery's power will eventually be used up. According to statistics, if a computer has never been turned on, the life the battery is from about a half year to one year. But in fact, the normal service life of a lithium cell is 10 years. When the power of the battery is too low or is used up, the data in the CMOS cannot be maintained, and thus, the computer cannot be. The battery must then be changed. A regular user may not know the function or existence of the battery. When the computer cannot be smoothly booted, the user probably thinks the quality of the computer is bad. A computer repair technicians is called to repair the computer. Frequently the results of repairing the computer is to replace the battery with a new one because the electrical capacity of the old battery is depleted. After a new battery is mounted, the data in the CMOS must be set anew, which will bother the user and increase maintenance cost. The method of providing power to the RTC should be improved.